1. Field of the Invention
This invention relates to methods and apparatus for cooling and heating semiconductor wafers during the production of integrated circuit structures. More particularly, this invention relates to wafer cooling and heating by means of a heat transfer gas.
2. Description of the Related Art
During the plasma etching of a semiconductor wafer, such as a silicon wafer, it is desirable to cool the wafer to avoid excessive heat buildup which can interfere with the processing. For example, excess heat can cause reticulation of the photoresist resulting in reduced sensitivity of the process. Other integrated circuit production processes such as Chemical Vapor Deposition (CVD) require that the wafer be heated to support the proper chemical deposition reactions.
Wafer cooling is conventionally accomplished using a silicone rubber heat transfer pad which is mounted between the wafer and a pedestal and which functions as a wafer support and heat sink. However, it has been found that the transfer of heat from wafer to the pedestal heat sink via the silicone robber pad is not always sufficient to dissipate the amount of heat generated during the plasma etching process. Tests and analysis indicate that the primary heat transfer mechanism is due to thermal conduction of gas rather than due to contact conduction from the wafer to the pedestal through the silicone robber pad.
The use of gas as a thermal conductor in semiconductor wafer process is known for transmitting heat either to or from a wafer. For example, King U.S. Pat. No. 4,251,762 discloses a method for cooling a wafer being bombarded by ions in an ion implantation apparatus wherein a wafer is clamped adjacent its periphery to a target block and a cooling gas with a high thermal conductivity such as nitrogen, neon, helium, or hydrogen is fed under pressure into the space between the wafer and the cooling block through an orifice in the cooling block.
Lamont U.S. Pat. Nos. 4,680,061 and 4,743,570 describe the use of a gas to conduct heat to or from a wafer from a heat exchange means which may comprise a heater for heating the wafer or a heat sink for cooling the wafer. A pressure plate seals the periphery of a wafer carrier plate assembly against the wall of a vacuum apparatus while the wafer is either heated or cooled by the heat exchange means utilizing gas conduction heat transfer by introducing a fraction of the argon gas employed for operation of the sputter deposition source directly into the space between the heat exchange means and the wafer.
However, these prior uses of a gas as a heat transfer conductor have been limited to use of the gas in a separate chamber or compartment of the apparatus isolated from the portion of the process apparatus generating the heat, e.g., the sputter deposition chamber, either because a different gas was used for the heat transfer than the gas used in the process, or because it is desired to maintain a higher vacuum in the main process apparatus than that of the cooling chamber. To maintain this separation and pressure differential, the wafer is usually clamped and/or sealed to provide this higher pressure on the backside of the wafer.
For example, in the aforementioned King patent, the pressure or vacuum in the ion implantation process chamber is said to be 7.times.10.sup.-7 Torr while the pressure behind the wafer is said to vary from 0.5 to 2.0 Torr. In the aforementioned Lamont patent the argon gas is said to be admitted into the heating station at pressures of 100 to 1000 microns, which pressures are stated to be one to two orders of magnitude higher than the normal argon pressure of 10 microns in the main chamber.
While it has been thought that the use of such high pressures (relative to the processing pressures) are necessary to achieve the desired heat transfer through the gas, quite surprisingly, it has been found that good heat transfer can be obtained or achieved at the same pressure or vacuum conditions used in carrying out the plasma etching process, thus making unnecessary the prior art practice of using high pressures and sealing between the wafer and the processing chamber.